Feedback and delayed CSI can be as good as perfect CSI

The degrees of freedom (DoF) region of the two-user MIMO interference channel (IC) is completely characterized in the presence of noiseless channel output feedback from each receiver to its respective transmitter and with the assumption of delayed channel state information (CSI) at the transmitters. It is shown that having output feedback and delayed CSI at the transmitters can strictly enlarge the DoF region when compared to the case in which only delayed CSI is available at the transmitters. Furthermore, cases are identified in which output feedback and delayed CSI alone are sufficient to achieve the DoF region achievable with perfect, instantaneous CSI.